Fire resistance for optically transparent thermoplastics

ABSTRACT

A fire retardant, transparent panel and method of making the panel, that is especially well suited for use on mobile platforms, and particularly on aircrafts. The panel is a composite of a transparent matrix and a plurality of fire-retardant nanoparticles. The nanoparticles have a diameter less than the visible spectrum of light, and in one form between about 0.1 nm to about 400 nm in diameter. The nanoparticles may be arranged randomly or in predetermined patterns within the matrix during manufacturing of the panel. The panel is lightweight, transparent and yet highly fire retardant.

FIELD

The present invention relates to composite transparencies and moreparticularly to composite transparencies utilized to provide improvedfire resistance for optically transparent thermoplastics that can beincorporated into a mobile platform.

BACKGROUND

Fire safety is important because of the destructive nature of anuncontrolled fire. Fire is a self-propagating cycle of reactions wherematter changes form as indicated by the visible and tangible sideeffects of heat, light, and gas emission. Fire is caused by raising afuel (such as plastic or wood, for example) to its combustiontemperature. The heat decomposes the complex molecules of the fuel intosmaller molecules that recombine with oxygen in the air to producedifferent fuels, generate more heat, and release various gases. Theburning fuel releases free radicals that emit light or produce a flame.The fuel decomposition and radical emission contribute to the fire cycleuntil only the nearly pure carbonaceous base of the fuel, known as char,remains. The fire is accompanied by the release of volatile gascompounds, or smoke, from the fuel components that do not char. As longas there is a source of oxygen, free radicals, and fuel, the fire willcontinue and may become uncontrollable.

Technologies such as flame retardants and flame resistant materials, forexample, have been developed to control fire by raising the combustiontemperature, reducing the rate of burning, reducing flame propagation,and reducing smoke generation. Flame retardants include inorganicminerals, phosphorous compounds, halogenated organic compounds, andothers for the inclusion in various fuels or substrates. The inorganicminerals reduce heat generated to below the combustion temperature ofthe fuel by endothermically decomposing and releasing water vapor upondecomposition. The phosphorous compounds stop fire by expeditingcharring of the outer layers of the fuel to insulate any remaining fuelfrom burning and to prevent the release of combustible gases. Thehalogenated organic compounds stop fire by eliminating the free radicalsthat contribute to the combustion process.

Choosing a flame retardant may be based on the desired mechanism ofprotection or may be based on non-fire related criteria as demonstratedin the transportation industry where the multiple safety variables areconsidered in order to best protect equipment and/or passengers. Inmobile platforms such as aircrafts, trains, buses, or ships that areoperator driven, it is necessary that visibility through transparenciesof the mobile platform such as windows does not interfere with thevisual range needed to safely maneuver the mobile platform. Whenselecting flame retardants for the transparency, particular care must beused because upon combustion of transparency polymers, the initialpolymer material burns to undergo crosslinking, aromatization, fusion ofaromatics, polymer chain scission, polymer chain stripping, and thelike. These changes in polymer structure prolong the burning, provideinnumerable fuel sources, increase flame propagation, and increase thegeneration of toxic gases.

Current methods of preventing fire and enhancing fire resistance oftransparencies have included incorporating phosphorous and halogenatedorganic compounds into the transparency material, for example, byapplying coatings of the materials onto the transparencies. Despite thebenefits, the phosphorous and halogenated organic compounds producecorrosive smoke and/or various environmentally detrimental and toxicemissions. Furthermore, the coated transparencies have a differentrefraction index (RI) than the uncoated transparency material causinglight bending and image distorting through the coated transparency orcause clouding and opacity of the transparent material.

Accordingly, there is still a need for a fire-resistant transparencythat can provide fire resistance, reduce flame spreading, reduce therate of heat release, facilitate extinction of the fire, minimize smokeevolution, and does not distort the refractive index of the transparencyor impair the visibility through the transparency. The present inventionis illustrated in connection with an aircraft window, however it isapplicable to any transparency where fire resistance and undistortedvisibility are of paramount importance.

SUMMARY

In various embodiments, the present invention provides a fire-retardanttransparent panel comprising a transparent matrix and a plurality offire-retardant nanoparticles integrated within the transparent matrix.The fire-retardant nanoparticles have a diameter less than thewavelength of visible light. The fire-retardant nanoparticles can be inparticulate form or various types of fibers. In various embodiments, thefire-retardant nanoparticles are endothermic materials or dehydrators,or are independently selected from alumina trihydrate, magnesiumhydroxide, calcium carbonate, ferrocene, and combinations thereof.

Methods for providing a fire-retardant transparent panel are alsoprovided. The methods generally comprise providing a transparent matrix,providing a plurality of fire-retardant nanoparticles having a diameterless than the wavelength of visible light, and integrating thefire-retardant nanoparticles within the transparent matrix such that thepanel is substantially insensitive to a difference between therefractive index of the transparent matrix and a refractive index of thefire-retardant nanoparticles. The arrangement of the fire-retardantnanoparticles in the transparent matrix can be selected to enhance theoverall fire-retardant properties of the panel and/or to minimizeimpacts of the nanoparticles on visibility.

An aircraft having a fire-retardant window is also provided. Theaircraft generally comprises a fuselage having an opening. A window isplaced in the opening. The window comprises a transparent panel and aplurality of fire-retardant nanoparticles having a diameter less thanthe wavelength of visible light.

The features, functions, and advantages can be achieved independently invarious embodiments of the present inventions or may be combined in yetother embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is an illustration of an exemplary mobile platform including afire-retardant transparent panel according to various embodiments of thepresent invention;

FIG. 2 is a sectional view of the fire-resistant transparent panel shownin FIG. 1;

FIG. 3A is a schematic view of an injection mold used to construct thefire-retardant transparent panel, shown in FIG. 1, in accordance withvarious embodiments of the present invention;

FIG. 3B is a schematic view of a fire-retardant nanoparticlepre-impregnated tape used to construct the fire-retardant transparentpanel, shown in FIG. 1, in accordance with various embodiments of thepresent invention; and

FIG. 3C is a sectional view of a fire-retardant transparent panelaccording to various embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiment(s) is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

Referring to FIG. 1, a fire-retardant transparent panel 10 according tovarious embodiments of the present invention is depicted in operativeassociation with a mobile platform 12. In various embodiments, themobile platform 12 is an aircraft. Although the mobile platform 12 isshown as an aircraft, the mobile platform 12 could also be representedin the form of other mobile platforms, including, but not limited to aship, a train, a bus, or an automobile. Additionally, although variousembodiments of the present invention will be described below asparticularly applicable for use in association with mobile platforms,the invention should not be so limited in application. It is envisionedthat the invention is equally applicable to aircrafts, trains, buses,ships, buildings, masks, respirators, goggles, or any application wherea fire-retardant panel having high transparency and uninterruptedvisibility is of paramount importance.

In the particular example provided, the fire-retardant transparent panel10 is shown as a window in an opening in a fuselage 14 of the mobileplatform 12. It should be appreciated, however, that the fire-retardanttransparent panel 10 can be used in any interior or exterior portion ofthe mobile platform 12, and may include the cockpit window or a doorwindow. Moreover, the fire-retardant transparent panel 10 can be used inany number of environments not strictly limited to conventional“windows”. For example, skylights, running light covers, satellite domecovers, view ports, and various other environments can employ thefire-retardant transparent panel 10 of the present invention.

FIG. 2 illustrates a portion of the fire-retardant transparent panel 10.The fire-retardant transparent panel 10 generally includes a pluralityof fire-retardant nanoparticles 16 integrated within a transparentmatrix 18. In a preferred form, the transparent matrix 18 is formed frompolycarbonate, a transparent epoxy resin, or polyvinyl fluoride. Anysuitable polymeric material may also be used for the transparent matrix18 including, but not limited to thermoplastics.

The fire-retardant nanoparticles 16 are materials that reduce the spreadof fire or prevent fire. Example fire-retardant materials include, butare not limited to, alumina trihydrate, magnesium hydroxide, calciumcarbonate, ferrocene, and combinations thereof. The fire-retardantnanoparticles 16 may be any other suitable material. For example,endothermic materials or materials that absorb heat generated by thesurroundings or dehydrators that remove sources of oxygen from a burningsystem may be employed. Suitable fire-retardant materials also includevarious transition metals. The fire-retardant materials may be selectedbased the desired level of heat control, the relative amount of thefire-retardant material for incorporation in the transparent matrix 18,the end use of the fire-retardant transparent panel 10, and anystructural and physical limitations of the system into which thefire-retardant transparent panel 10 is incorporated. While not intendingto be bound by a particular theory, it is believed that selectfire-retardant nanoparticles 16 reduce fire by endothermic means suchthat when the fire-retardant material combusts, it absorbs heat from thesurroundings, thereby reducing the temperature of the fire. After asuccession of these endothermic reactions, the fire becomes extinguishedas the combustion temperature of the fuel cannot be maintained by theburning system.

Although the fire-retardant nanoparticles 16 are depicted as individualfibers, the fire-retardant nanoparticles 16 can be of any suitable shapeincluding particles, continuous fibers, chopped fibers, bundled fibers,intertwined fibers, and combinations thereof. In embodiments utilizingbundled fibers or weaved fibers, or fibers where there is overlap, it isunderstood that the total thickness of the overlapped regions is to notexceed the wavelength of light as to prevent any interference withvisibility due to light scattering or refraction from the fibers.

The fire-retardant nanoparticles 16 can also be a mixture of fibers andparticles. The integration of the fire-retardant nanoparticles 16 can betailored to extinguish the fire in a distinct pattern or a time frame.For example, a dense region of fire-retardant particles can be placed onthe outer surfaces of the transparent panel 10 while a single continuousfiber loops the core of the transparent panel. The dense region ofparticles could cause the surface of the fire-retardant transparentpanel 10 to char to quickly block the remaining fuel source and stop thespread of fire. In other embodiments, dense regions could be used toisolate the fire. For example, strips of the fire-retardant transparentpanel 10 could have a dense population of the fire-retardantnanoparticles 16 such that the fire is blockaded and cannot spread toother regions of the fire-retardant transparent panel 10.

In one embodiment the fire-retardant nanoparticles are in fiber form andcomprise alumina trihydrate. The alumina trihydrate is prepared using asolution synthesis or a precipitation method. An aluminum source, suchas aluminum sulfate, is combined with a solution that provides therequisite oxidation of the aluminum sulfate and/or provides thehydrating component until the aluminum oxide trihydrate or aluminatrihydrate begins to precipitate from the solution. The solids can beremoved to stop precipitation. Increasing the length of time theprecipitate remains in the solution leads to a greater diameter of thenanofiber or particulate or a longer nanofiber, depending on theagitation of the system and other reaction controls such as temperature.Additional synthesis techniques include laser induced dehydroxylation ofAl(OH)₄ ⁻ to Al(OH)₃, hydrolysis of Al(CH₃)₃ in a turbulent subsonic gasjet reactor, sonochemical processing, microemulsion processing,high-energy ball milling, or any other techniques known in the art usedto produce nanoparticulates.

The dependence of light scattering upon particles in a coating can bedescribed by the following relationship:I_(s)α(Nd⁶/λ⁴)(Δn)²,

where N is the total number of nanoparticles, d is the diameter of thenanoparticles or nanoparticle aggregates, λ is the wavelength of lightthat is incident upon a coating with the nanoparticles, and Δn is thedifference in the index of refraction of the nanoparticles and thetransparent matrix. In a one embodiment the diameter “d” of thefire-retardant nanoparticles 16 is less than the wavelength of visiblelight, i.e., less than from about 400 nm to about 600 nm. Preferably,the fire-retardant nanoparticles 16 have a diameter d of from about 0.1nm to about 400 nm, and more preferably from about 1 nm to about 5 nm.In embodiments where the fire-retardant nanoparticles 16 are in fiberform, the fibers can range from about 10 nm in length to about 500 nm inlength. The small diameter insures that any light absorbed by thealumina trihydrate will be in the ultraviolet range and thereforeinvisible to the human eye.

Preferably the fire-retardant nanoparticles 16 are distributed withinthe transparent matrix 18 at from about 1% by volume to 60% by volume,and more preferably from about 20% by volume to about 40% by volume.

In accordance with a preferred implementation of the present invention,due to the diameter d being less than the wavelength of visible light,the fire-retardant nanoparticles 16 provide an uninterrupted view. Thetransmittance is on the order of at least 90% and preferably at leastabout 95%. Moreover, because the light absorption of the fire-retardantnanoparticles 16 is in the ultraviolet range and not visible to thehuman eye, it is possible to allow dissimilar RIs between thefire-retardant nanoparticles 16, such as the white pigment aluminatrihydrate, and the transparent matrix 18, such as polycarbonate,without the fire-retardant transparent panel 10 becoming opaque.

In an exemplary implementation, the fire-retardant nanoparticles 16 areintegrated with the transparent matrix 18 utilizing an injection moldingprocess as illustrated in FIG. 3A. A mold 20 generally includes moldhalves 22 that combine to form a designated shape, such as, for example,a window shape. The epoxy resin 24 and fire-retardant nanoparticles 16are then injected into the mold 20. The fluid flow of the epoxy resin 24can be varied to direct mixing and direction of the nanoparticles 16. Inan alternate preferred embodiment, not depicted, the inlets for theepoxy resin 24 and the fire-retardant nanoparticles 16 can be on thesame side of the mold half 22. A cycle of higher and slower flow ratesof the epoxy resin 24 can also be implemented to achieve proper mixingof the epoxy resin 24 and the fire-retardant nanoparticles 16 orselectively deposit the fire-retardant nanoparticles 16 across the panel10. It is understood that the epoxy resin 24 and the fire-retardantnanoparticles 16 may be mixed prior to disposition of the mixture intothe mold half 22 or after the components are placed in the mold half 22.Once the epoxy resin 24 has set or cured, the fire-retardant transparentpanel 10 may then be removed from the mold 20. It is possible for themold 20 to take on any shape desired, thereby allowing windows that havecomplex surfaces.

Turning to FIG. 3B, in an alternative preferred embodiment, thefire-retardant nanoparticles 16 are used to form a reinforcedpre-impregnated tape. For example, the fire-retardant nanoparticles 16may be arranged in a resin that, after solidification, forms thetransparent matrix 18 in the form of strips of pre-impregnated tape 26.Successive layers of the pre-impregnated tape 26 may then be laminatedto form the fire-retardant transparent panel 10. Due to the randomorientation of the fire-retardant nanoparticles 16 on thepre-impregnated tape 26 of various embodiments, the pre-impregnated tape26 may not need to be aligned in any particular manner when laminatedwith layers of other pre-impregnated tape 26 to form the fire-retardanttransparent panel 10.

In addition to the random orientation of the injection molding, theintegration of the fire-retardant nanoparticles 16 can also be in anordered form such as lined patterns, such as strips or grids, regularshaped patterns, such as circles, rectangles, squares, or ellipses,irregular shaped patterns or free-form patterns, overlapping patterns,such as an interconnected series of circles, weaved patterns, such ascloth, and various combinations thereof. As depicted in FIG. 3C, thefire-retardant nanoparticles 16 are arranged in a substantially linearpattern. The linear pattern may be achieved by forming a laminate of thetransparent matrix 18 and the nanoparticles 16. The pressure anddirectional force resultant from the laminate process can be manipulatedto align the nanoparticles 16 in rows. In various embodiments,pultrusion techniques that generally include pulling a fiber reinforcedresin through a shaping die, can be used to orient the fire-retardantnanoparticles 16 and provide the desired linear or regular shapedpattern.

Regardless of the particular technique employed, it is desirable thatthe implementation of the fire-retardant nanoparticles 16 be performedto prevent clumping or undesired aggregation of the fibers. In variousembodiments, it may be desirable to integrate the fire-retardantnanoparticulates 16 in a manner to provide dense regions or a gradientof the fire-retardant nanoparticles 16.

By employing fire-retardant nanoparticles 16 having a diameter less thanthe wavelength of light integrated within a transparent matrix 18, thefire-retardant transparent panel 10 is substantially insensitive to RI‘mismatch’, e.g. ‘mismatch’ caused by the integration of materialshaving different refractive indices. This surprising result isespecially beneficial because some fire-retardant materials are actuallypigments, such as the white coloring of alumina trihydrate. Thefire-retardant nanoparticles 16 prevent light scattering and refractionthat occurs from particles that can deflect light. That is, thefire-retardant transparent panel 10 will maintain a high level oftransparency, e.g. 90% or greater, in the presence of the fire-retardantnanoparticles 16. Moreover, the fire-retardant transparent panel 10 hasan increased hardness which can be useful in the mobile platform 12.

Preferably, the fire-retardant nanoparticles 16 are incorporated intothe fire-retardant transparent panel in an amount sufficient to provideat least one of flame spread reduction, heat reduction, smoke reduction,rapid charring, and safety to the panel and its surroundings. Theconcentration of the fire-retardant nanoparticles incorporated can bealtered based on the end use of the fire-retardant transparent panel,for example, maximizing the concentration of flame-retardantnanoparticles 16 to provide flame resistance to distinct regions of thefire-retardant transparent panel 10 where the flame may be able topropagate more as compared to other regions.

While various preferred embodiments have been described, those skilledin the art will recognize modifications or variations, which might bemade without departing from the inventive concept. The examplesillustrate the invention and are not intended to limit it. Therefore,the description and claims should be interpreted liberally with onlysuch limitation as is necessary in view of the pertinent prior art.

1. A method for providing a fire-retardant transparent panel comprising:providing a transparent matrix; providing a plurality of opticallytransparent, fire-retardant nanoparticles having a diameter less thanthe wavelength of visible light, and with an elongated shape;integrating the fire-retardant nanoparticles within the transparentmatrix to form the fire-retardant transparent panel such that thefire-retardant nanoparticles remain in the transparent matrix in theiroriginal form, and such that the panel is substantially insensitive to adifference between a refractive index (RI) of the transparent matrix anda RI of the fire-retardant nanoparticles; and wherein said integratingthe fire-retardant nanoparticles within a transparent matrix comprisesintegrating the fire-retardant nanoparticles within a transparent matrixin an orientation selected from the group consisting of randomorientation, lined patterns, regular shaped patterns, irregular shapedpatterns, overlapping patterns, weaved patterns, and combinationsthereof.
 2. The method of claim 1, wherein the fire-retardantnanoparticles and the transparent matrix are injected into a mold toform the fire-retardant transparent panel.
 3. The method of claim 1,wherein providing the plurality of fire-retardant nanoparticlescomprises providing the fire-retardant nanoparticles having a diameterfrom about 0.1 nm to about 400 nm.
 4. The method of claim 1, wherein thefire-retardant nanoparticles are selected from the group consisting ofalumina trihydrate, magnesium hydroxide, calcium carbonate, ferrocene,and combinations thereof.
 5. The method of claim 1, wherein integratingthe fire-retardant nanoparticles within a transparent matrix comprisesintegrating the fire-retardant nanoparticles such that thefire-retardant nanoparticles comprise from about 1% by volume to 60% byvolume of the fire-retardant transparent panel.
 6. A method for forminga fire-retardant transparent panel, comprising: providing a transparentmatrix; providing a plurality of optically transparent, fire-retardantnanoparticles each having a diameter less than the wavelength of visiblelight; selecting said nanoparticles from at least one of aluminatrihydrate, magnesium hydroxide, calcium carbonate and ferrocene;integrating the fire-retardant nanoparticles within the transparentmatrix to form the fire-retardant transparent panel such that the panelis substantially insensitive to a difference between a refractive index(RI) of the transparent matrix and a RI of the fire-retardantnanoparticles; and wherein said providing a plurality of fire-retardantnanoparticles comprises providing a plurality of fire-retardantnanoparticles that each have at least one of a diameter from about 0.1nm to about 400 nm and an elongated shape, and wherein thefire-retardant nanoparticles remain in their original form after beingintegrated into the transparent matrix.
 7. The method of claim 6,wherein selecting said nanoparticles comprises selecting saidnanoparticles from a combination of said alumina trihydrate; magnesiumhydroxide, calcium carbonate and ferrocene.
 8. The method of claim 6,wherein integrating the fire-retardant nanoparticles comprisesintegrating the fire retardant nanoparticles within a transparent matrixin a random orientation.
 9. The method of claim 6, wherein integratingthe fire-retardant nanoparticles comprises integrating the fireretardant nanoparticles within a transparent matrix in lined patterns.10. The method of claim 6, wherein integrating the fire-retardantnanoparticles comprises integrating the fire retardant nanoparticleswithin a transparent matrix orientation in regular shaped patterns. 11.The method of claim 6, wherein integrating the fire-retardantnanoparticles comprises integrating the fire retardant nanoparticleswithin a transparent matrix orientation in irregular shaped patterns.12. The method of claim 6, wherein integrating the fire-retardantnanoparticles comprises integrating the fire retardant nanoparticleswithin a transparent matrix orientation in overlapping patterns.
 13. Themethod of claim 6, wherein integrating the fire-retardant nanoparticlescomprises integrating the fire retardant nanoparticles within atransparent matrix orientation in weaved patterns.
 14. The method ofclaim 6, wherein integrating the fire-retardant nanoparticles comprisesintegrating the fire retardant nanoparticles within a transparent matrixorientation in combinations of differing patterns.
 15. A method forforming a fire-retardant transparent panel, comprising: providing atransparent matrix; providing a plurality of optically transparent,fire-retardant nanoparticles each having a diameter between about 0.1 nmand 400 nm and an elongated shape; and integrating the fire-retardantnanoparticles within the transparent matrix in a predeterminedorientation to form the fire-retardant transparent panel such that thepanel is substantially insensitive to a difference between a refractiveindex (RI) of the transparent matrix and a RI of the fire-retardantnanoparticles, and such that the fire-retardant nanoparticles remain intheir original form after being integrated within the transparentmatrix.
 16. The method of claim 15, wherein the fire-retardantnanoparticles are selected from the group consisting of aluminatrihydrate, magnesium hydroxide, calcium carbonate, ferrocene, andcombinations thereof.
 17. The method of claim 15, wherein integratingthe fire-retardant nanoparticles comprises injecting said fire-retardantnanoparticles and said transparent matrix into a mold to form saidfire-retardant panel.
 18. The method of claim 15, wherein integratingthe fire-retardant nanoparticles within the transparent matrix comprisesintegrating the fire-retardant nanoparticles within the transparentmatrix in at least one of: a random orientation; in a lined pattern; ina regular shaped pattern; in an irregular shaped pattern; in overlappingpatterns; and in weaved patterns.